This invention relates to a simple, inexpensive timed fast-rate battery charger. More particularly, it relates to an improved timed, thermostatically controlled fast charger which supplies charging current to a battery load at a fast charge rate for a predetermined time followed by a continuous slow charge rate.
An increasing number of consumer products are operated by one or more rechargeable cells, such as nickel-cadmium cells. These cells are available in many different physical sizes with various electrical charging characteristics. Typical nickel-cadmium cells are capable of being charged at a very high rate.
If the cell characteristics are known, and the state of charge of the cell is known, a timed charge of extremely high rate can be safely put into the cell without risk of permanent damage. Even if the state of charge is not known, it is still possible to safely and reliably inject a significant amount of charge (less than in the known discharged state) at a high rate.
The timed high-rate charge can be applied as an exclusive charge method or can be followed with a slow-rate charge. In the timed approach, a constant-current charging source of appropriate output is connected to the cell through a timed switch. Conventional timed chargers utilize mechanical, thermal, electrical, or even chemical timing methods to control the duration of the fast-rate charging current. Once the timer is actuated, the fast-rate charging current is fed to the cell for a predetermined time, and then interrupted. For example, a completely discharged 1.0 ampere-hour cell in a given application may be fast-charged safely at its 5 C rate, 5.0 amperes, for up to 10 minutes before the timer cuts off its fast charge. The timed-only charge system works best when, in normal use, the device presents an essentially discharged cell to the charger. Under such conditions, the time and rate can be selected to provide a charge which will utilize a significantly high percentage of the cell capacity.
Where the charger application presents a high probability that partially-charged or even fully-charged, cells will be connected to the charger, then the charge (current-time product) must be reduced to a value which a fully charged cell can tolerate without unacceptable compromise of operating life.
Split-rate charging techniques are often used in conjunction with temperature-controlled fast charge rate systems. In the split-rate charging technique utilizing temperature control, the battery is initially charged at a fast charge rate. At an appropriate time, generally at or about the time the cell achieves an overcharged state, the charging rate is switched to a slow-rate. Ideally, the slow charging rate is low enough for the battery to withstand being overcharged at that rate indefinitely.
The primary advantage of the conventional split-rate charging technique is the ability to partially charge a battery in a very short time, often in a matter of several minutes. An important requirement associated with this technique, however, is the necessity of providing some sort of charge control means which effects the transition from a fast charging rate to a slow charging rate if full charge is approached during fast charging. Typically, the charge control means comprises a sensor which may be activated by either voltage, temperature, pressure or a combination thereof.
One type of known split-rate charger is the thermostatically controlled fast charger. Conventionally, this device consists of a constant current power supply and a battery containing a thermostat. The thermostat is usually attached to, or placed in the vicinity of, a cell in the battery and responds to the temperature rise in the cell itself as it approaches the fully charged state. Initially the thermostat is closed to provide a fast charge rate current path. At a given temperature, it automatically opens, to disconnect the fast charge rate current path. Thereafter, the charger normally switches to a slow charge rate. As a result of the problem of oxygen and/or hydrogen generation when charge rates exceeding 4 C are utilized to attain a temperature rise in the cell adequate to operate the thermostat reliably, such chargers are limited to operation at charge rates not exceeding 1.5 C (40 minutes). For many applications this charge rate limitation may be significant.
Another conventional approach provides timed fast charging by utilizing the heat generated by the fast charge rate current flowing through a low value resistor to control the operation of the thermostat. At a given temperature, the thermostat automatically opens, and switches a high value resistor shunted across the thermostat in series with the low value resistor. Thereafter, the series combination of the two resistors provides a slow charge rate current path. A primary disadvantage in the approach is that the heat power input to the thermostat from the low value resistor is proportional to the square of the fast charge current. This means that for a timing system, essentially adiabatic during fast charging, the charge input to the battery would increase linearly with time, whereas the fast charge time to the opening of the thermostat would be inversely proportional to the square of the current. Thus, variation of unregulated charge current over a range of line voltage and battery voltage would be a major contributor to charge input variability. Again, this drawback may be significant in many applications.
It is a general object of the present invention to provide a split-rate charger which is not accompanied by the limitations and drawbacks, outlined above, associated with conventional thermostatically timed fast chargers.
It is a further object of the invention to provide a simple, low-cost charger for providing a sequential timed fast-rate charging mode and a slow-rate charging mode. The chargers in accordance with the present invention provide a reliable switching mode between charging rates, and do so at costs which make them useful in the consumer market where economy of manufacture is so important.